Graded core/shell semiconductor nanorods and nanorod barcodes

ABSTRACT

Graded core/shell semiconductor nanorods and shaped nanorods are disclosed comprising Group II-VI, Group III-V and Group IV semiconductors and methods of making the same. Also disclosed are nanorod barcodes using core/shell nanorods where the core is a semiconductor or metal material, and with or without a shell. Methods of labeling analytes using the nanorod barcodes are also disclosed.

PRIORITY

This application is a divisional of U.S. patent application Ser. No.10/659,992, filed Sep. 10, 2003, which claims priority to U.S.Provisional applications 60/409,843, filed Sep. 10, 2002 and 60/409,845,filed Sep. 10, 2002, the contents of which are all incorporated hereinby reference for all purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

The invention described and claimed herein was made in part utilizingfunds supplied by the United States Department of Energy under contractNO. DE-AC03-76SF000-98 between the United States Department of Energyand The Regents of the University of California. The government hascertain rights to the invention.

BACKGROUND OF THE INVENTION

Colloidal semiconductor nanocrystals is an important field in modernnanoscale science and technology, see Bawendi, M. G.; Steigerwald, M.L.; Brus, L. E. Annu. Rev. Phys. Chem. 1990, 41, 477-496 and Alivisatos,A. P. Science 1996, 271, 933-937, the contents of both are herebyincorporated by reference in their entirety for all purposes. Among thevarious materials, colloidal CdSe quantum dots are undoubtedly the moststudied, due to their tunable emission in the visible range, theadvances in their preparation and their potential use in industrial andbiomedical applications.

Recently, several advances in the synthesis of colloidal semiconductornanocrystals have been made, allowing for size and shape control, seePeng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich,A.; Alivisatos, A. P. Nature 2000, 404, 59-61 and Manna, L.; Scher, E.C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700-12706, thecontents of both are hereby incorporated by reference in their entiretyfor all purposes. Of particular interest in this respect is the abilityto obtain quantum confined wurtzite CdSe nanorods with a narrowdistribution of lengths and diameters. Well-characterized samples ofCdSe nanorods have become a model system to study theories of quantumconfinement: for instance, it has been demonstrated, both theoreticallyand experimentally, that they emit linearly polarized light along thec-axis and that the degree of polarization is dependent on the aspectratio of the particles. Semiconductor nanorods are of particularinterest because of their possible applications in light emittingdiodes, in low-cost photovoltaic devices, their propensity to formliquid crystalline phases and their use as barcodes for analyticalpurposes.

U.S. Pat. No. 6,225,198, the contents of which are hereby incorporatedby reference in its entirety for all purposes discloses processes forforming Group II-VI semiconductor nanocrystals and rod-like structuresby contacting the semiconductor nanocrystal precursors with a liquidmedia comprising a binary mixture of phosphorous-containing organicsurfactants. In semiconductor quantum dots, which are nanocrystals andnot the nanorods of the present invention, high emission efficiency fromband-edge states is required to study in detail their electronicstructure or more practically, if they are to be used as emitters in anyapplication. Unfortunately, the band-edge emission from nanocrystals hasto compete with both radiative and non-radiative decay channels,originating from surface electronic states. In colloidal nanocrystals,coating the surface of the nanocrystals with suitable organic moleculescan minimize this problem. The judicious choice of a passivating agentcan in fact improve the size-dependent band-edge luminescenceefficiency, while preserving the solubility and processability of theparticles. Unfortunately, passivation by means of organic molecules isoften incomplete or reversible, exposing some regions of the surface todegradation effects such as photooxidation. In some cases, chemicaldegradation of the ligand molecule itself or its exchange with otherligands might lead to unstable and therefore unusable nanocrystals.

In the case of colloidal CdSe nanorods, there are two additional factorsthat might further reduce the luminescence from band-edge states, whencompared to spherical CdSe nanocrystals. In nanorods, thesurface-to-volume ratio is higher than in spheres, and this increasesthe occurrence of surface trap-states. In larger dots, the increaseddelocalization of carriers reduces the overlap of the electron and holewavefunctions, lowering the probability of radiative recombination. Thedelocalization of carriers should be particularly high in a nanorod,where they are free to move throughout the length of the rod, therebyleading to reduced luminescence in nanorods. In order to efficiently andpermanently remove most of the surface states of the nanocrystal, aninorganic material can be epitaxially grown on its surface, see Peng, X.G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem.Soc. 1997, 119, 7019-7029, the contents of which is hereby incorporatedby reference in their entirety for all purposes.

A stringent requirement for the epitaxial growth of several monolayersof one material on the top of another is a low lattice mismatch betweenthe two materials. If this requirement is not met, a strain accumulatesin the growing layer and eventually it may be released through theformation of misfit dislocations, degrading the optical properties ofthe system, see for example Dabbousi, B. O.; RodriguezViejo, J.;Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.;Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463-9475, the contents ofwhich is hereby incorporated by reference in their entirety for allpurposes. The preparation of a coated semiconductor nanocrystal may befound in U.S. Pat. Nos. 6,607,829 and 6,322,901 the contents of whichare all incorporated by reference in their entirety for all purposes.

In the case of “spherical” colloidal CdSe nanocrystals, there are twomethods of efficient inorganic passivation, one by means of a sphericallayer (or shell) of ZnS, and the other by means of a shell of CdS. Thechoice of these materials is based on the fact that both ZnS and CdSprovide a potential step for electrons and holes originating in thenanocrystals, reducing the probability for the carriers to sample thesurface. Surprisingly, the requirement for a low lattice mismatch is notas stringent as for 2D systems, because the total area over which thestrain accumulates is small, and the total strain energy at theinterface can remain below the threshold for inducing dislocations. Theextended surface of the CdSe rods has an average curvature that isintermediate between the surface of a spherical dot and that of a flatfilm. In addition, since CdSe nanorods can be produced with lengthsranging from a few nanometers to a hundred nanometers, the coherentgrowth of an epitaxial shell over a region that is much more extendedthan the surface of a spherical dot is more challenging. Both conditionsimply that interfacial strain will play a much more important role inrods than in dots. An additional issue that must be taken into accountis the solubility of the resulting particles. The shell growth must becarried out in a surfactant that provides surface accessibility for theshell material to grow, while preventing aggregation of the particles.The temperature must also be kept low enough to prevent nucleation ofthe shell material, while high enough that the surfactant is dynamicallygoing on and off the nanocrystal surface allowing access to themonomers.

The art is replete with chemical and biological assays to identify aparticular analyte of interest. Examples included immunoassays,fluorescence, signal amplification, nucleic acid hybridization and highthroughput screening. Each of the above-described assay formats utilizesdetectable labels to identify the analyte of interest. Radio-labeledmolecules and compounds are frequently used to detect biologicalcompounds both in vivo and in vitro. However, due to the inherentproblems associated with the use of radioactive isotopes,non-radioactive methods of detecting biological and chemical compoundsare often preferable. U.S. Pat. No. 6,274,323 (the contents of which arehereby incorporated by reference in its entirety for all purposes)discloses the use of semiconductor nanocrystals, or quantum dots in abarcode system for identification. The disadvantage of this technologyis that multiple quantum dots are required to perform an assay.

SUMMARY OF THE INVENTION

The invention described herein solves the aforementioned problems withthe prior art by disclosing graded core/shell semiconductor nanorods andbarcode nanorods. A graded shell of larger band gap is grown around asemiconductor rod using a surfactant, in some cases tributylphosphine.Interfacial segregation is used to preferentially deposit onesemiconductor material near the core, providing relaxation of the strainat the core/shell interface. The invention allows for variation of theshell thickness by growing a desired number monolayers on core nanorodsranging from aspect ratios of 2:1 to 10:1. The current invention alsocontemplates a photochemical annealing process which provides core/shellnanorods having increased quantum efficiencies and having stability inair under visible or UV light. The inventors have surprisingly foundthat by “photoannealing” or “photochemical annealing” an unexpectedincrease in photoluminescence QY in core-shell rods results.

In another embodiment, the present invention contemplates gradedcore/shell semiconductor nanorods that comprise a core of a Group II-VI,a Group III-V or a Group IV semiconductor, and a graded shell comprisingthe same or different Group II-VI, a Group III-V or a Group IVsemiconductor.

In yet another embodiment of the present invention, there is a methodfor growing nanorods having alternating materials. The resultant “tails”or “nanorod barcodes” have use as detectable labels in various chemicaland biological applications. The cores of the nanorod barcodes havemetal or semiconductor cores.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows transmission electron micrographs (TEMs) of the mediumlength (3.3×23 nm) CdSe core nanorods (a) and the same cores withdifferent thickness shells of CdS/ZnS (b-d). The shell thickness is 2monolayers (b), 4.5 monolayers (c), and 6.5 monolayers (d).

FIG. 2 shows transmission electron micrographs (TEMs) of the short(5.0×18 nm) CdSe core nanorods (a) and the same cores with a CdS/ZnSshell (b). TEMs of the long (4.5×36 nm) CdSe core nanorods (c) and thesame cores with a CdS/ZnS shell (d).

FIG. 3 shows high resolution transmission electron micrographs (HRTEMs)of the medium length (3.3×23 nm) CdSe core nanorods (a) and the samecores with different thickness shells of CdS/ZnS (b-d). The shell growthis epitaxial with fringes going through both core and shell. The shellgrowth is also regular except for the thickest sample (d) where thestrain of the ZnS becomes too great and is relieved by irregular growthof ZnS on the shell.

FIG. 4 shows powder x-ray diffraction (XRD) patterns of the mediumlength (3.3×23 nm) nanorod cores (b) and core/shells (c-e). The bulk XRDpattern of CdSe (a) and ZnS (f) are given for reference. The initial XRDpattern of the CdSe core nanorods (b) match the peak positions of bulkCdSe but the intensities are different. The 002 peak is very narrow andmore intense than the other peaks because of the extended domain alongthe c-axis of the nanorods. In the thin (c) and medium (d) shell samplesall of the peaks shift and the intensity of the 002 peak decreasesrelative to the other peaks. In addition to following the aforementionedtrends, the thick shell sample (e) displays some small broad peakscorresponding to the ZnS growths and tails observed on them in the TEM.

FIG. 5 shows energy dispersive X-ray (EDX) spectra of the medium length(3.3×23 nm) nanorod cores and core/shells. The Se Kα spectrum is notshown as it was used for normalization of all the samples and it doesnot differ between them. The Cu Kα line was subtracted when calculatingthe Zn Kα area. It is very clear that there is no Zn or S in the corenanorods. The amount of these elements increases as a function of shellthickness. The Cd L lines are present in the initial CdSe core samplebut also increase as a function of shell thickness due to the growth ofCdS in the shell.

FIG. 6 shows absorption spectra (solid line) of medium length (3.3×21nm) CdSe core nanorods (a) and thin (b) and medium (c) core/shellsamples. Photoluminescence (PL) spectra (broken line) of the samesamples after photoannealing. The absorption spectra do not change uponphotoannealing.

FIG. 7 shows QY of medium length (3.3×21 nm) CdSe core nanorods (♦) andmedium core/shell (□) samples as a function of photonsabsorbed/nanocrystal (a). The cores do not change significantly withtime, but the core/shell's QY increases significantly after absorbing˜10⁹ photons/nanorod and then remains constant. Two PL spectra from thesame core/shell sample are shown in (b). The initial, non-photoannealedsample (o) was multiplied by 6.12 to match the intensity of the finalphotoannealed sample (−). There are no noticeable changes in the peakshape, peak maximum, or full width half max after photoannealing thesample.

FIG. 8 describes semiconductor nanorod barcode having two segments,without a graded shell.

FIG. 9 describes a semiconductor nanorod barcode with three segments,without a graded shell.

FIG. 10 describes a semiconductor nanorod barcode with two segments,without a graded shell, where one segment has a smaller cross sectionalarea than the first segment.

FIG. 11 describes a semiconductor nanorod barcode with three segments,each having a different cross sectional area.

FIG. 12 describes a semiconductor nanorod barcode with two segmentshaving a graded shell on one of the segments.

FIG. 13 describes a semiconductor nanorod barcode with two segments,both having a graded shell.

FIG. 14 describes a semiconductor nanorod with a graded shell, havingthree segments, each having a different cross sectional area.

FIG. 15 describes a nanorod barcode where one segment is a metal, andthe segment grown out of both ends comprises a semiconductor material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one embodiment of the present invention there is disclosed a gradedcore/shell semiconductor nanorod comprising at least a first segmentcomprising a core comprising a Group II-VI, Group III-V or a Group IVsemiconductor, a graded shell overlying the core, wherein the gradedshell comprises at least two monolayers, wherein the at least twomonolayers each independently comprise a Group II-VI, Group III-V or aGroup IV semiconductor. In another embodiment the graded core/shell hasat least three monolayers, and the monolayer closest to the corecomprises a first semiconductor material, and the outermost monolayercomprises a second semiconductor material, wherein between the monolayerclosest to the core and the outermost monolayer there exists aconcentration gradient of the first and second semiconductor material.The number of monolayers may between two and eight or 2 and 6. In oneembodiment there is a tail extending longitudinally from the core. In apreferred embodiment the core may comprise CdSe and the gradedcore/shell comprises CdS/ZnS. In yet another embodiment there is joinedto the first segment a second segment comprising a core comprising aGroup II-VI, Group III-V or a Group IV semiconductor, a graded shelloverlying the core, wherein the graded shell comprises at least twomonolayers, wherein the at least two monolayers each independentlycomprise a Group II-VI, Group III-V or a Group IV semiconductor. In apreferred embodiment the the second segment core comprises CdSe and thesecond segment graded shell monolayers comprise, in order, CdS/ZnS. Inanother embodiment the first and the second segments have differentcross sectional areas. The present invention also contemplates thatthere is a third segment joined to the second segment and the first,second and third segments have different cross sectional areas.

In another embodiment of the invention a nanorod barcode is disclosed.The barcode comprises a first segment of a first material and a secondsegment of a second material joined longitudinally to said firstsegment; wherein the at least one of the first and second segment iscapable of generating emission in response to excitation energy. In anembodiment the first and second segments comprise a nanorod core andsaid first and second segment cores independently comprises either asemiconductor material selected from the group consisting of GroupII-VI, Group III-V and Group IV semiconductors or a metal selected fromthe group consisting of transition metals, oxides and nitrides thereof.The invention also contemplates that there is a third segment connectedlongitudinally to said first segment core, and said third segment corecomprising a semiconductor material selected from the group consistingof Group II-VI, Group ITT-V and Group IV semiconductors. In oneembodiment, the second and third segments have different cross sectionalareas. In a preferred embodiment the first segment core comprises Co,and said second and third segment cores comprise CdSe. The inventioncontemplates that said first and second segments have different crosssectional areas. In one embodiment the at least one of said first andsecond segment cores have a graded shell overlying the core. In yetanother embodiment both segment cores have a graded shell overlying saidcores. In another embodiment there is a third segment joinedlongitudinally to said second segment, and said third segment comprisesa semiconductor material selected from the group consisting of GroupII-VI, Group III-V and Group IV semiconductors. In another embodimentthe at least one of said first and second and third segment cores have agraded shell overlying the core. In another embodiment all segment coreshave a graded shell overlying the cores. In yet another embodiment saidfirst, second and third segments have different cross sectional areas.

In another embodiment of the present invention there is disclosed amethod of growing a CdS/ZnS graded shell, comprising: providing a core,combining the core with at least one surfactant, heating the mixture,combining the mixture with a CdS/ZnS stock solution, wherein the corecomprises a semiconductor material, and graded core/shell nanorods areproduced. Preferably the core is rod shaped and comprises CdSe. In oneembodiment the mixture is heated to a temperature between 100-360° C.Preferably the mixture is heated to a temperature of 160° C. Preferablythe core is combined with only one surfactant. Preferably the surfactantis chosen from the group consisting of TOPO, TBP, HDA, HPA and TDPA. Inone embodiment the mixture is kept at a temperature of approximately160° for between 5 minutes and 24 hours after combining the CdS/ZnSstock solution, preferably the mixture is kept at a temperature of 160°C. for 10 minutes after combining the CdS/ZnS stock solution. Preferablythe core is a shaped nanorod. More preferably the core has a tetrapodshape. In a preferred embodiment the graded core/shell nanorods arephotochemically annealed, preferably using an Ar+ laser.

In another embodiment of the present invention there is disclosed amethod of using a nanorod barcode to identify an element, comprisinglabeling at least one identifiable element with at least one nanorodbarcode as described herein.

DEFINITIONS

By “thin, medium and thick shells”, it is meant that the thickness ofthe shells is approximately 2, 4.5 and 6.5 monolayers thick,respectively. One skilled in the art will appreciate that there is nosuch thing as 0.5 monolayer, and that the numbers 4.5 and 6.5 are astatistical average.

By “interfacial growth” it is meant growth of a crystal epitaxially ontoanother crystal forming an interface.

By “interfacial segregation” it is meant that the strain caused by thelattice mismatch of different crystals grown on top of each other causesthe reorganization of the shell to form a graded shell, one where theshell naturally goes from small to larger lattice mismatch.

By “strain and induced interfacial strain” it is meant that twodifferent crystal structures when grown epitaxially on to of one anotherinduce strain at the interface between them.

By “tail” it is meant that segment of material extends longitudinallyfrom one or both ends of a nanorod, or shaped nanorod. Two or moresegments of materials joined together as a nanorod is also termed a“nanorod barcode”. The “tails” themselves are nanorods. One skilled inthe art will appreciate that for a nanorod not rod shaped, i.e. onehaving arms, such as tetrapods, that tails may be grown out of either orall ends of the nanorods, providing the cores are not semiconductormaterial.

By “joined” it is meant chemically bonded.

By “core” it is meant a shaped material that is not spherical. Theinvention contemplates that the core is rod-like, and may have othercomplex shapes as described herein. The core may comprise asemiconductor material or other metals, alloys, nitrides, oxides, etc.as defined herein.

By “transition metals” it is meant elements with atomic numbers 21-30,39-48 and 57-80.

By “overlying said core” it is meant that the shell at least partiallycovers the core. In a preferred embodiment the shell covers the coreentirely.

By “monolayer” it is meant a single layer of atoms in one plane. Theatoms may be the same or different. The invention contemplates no limitto the amount of monolayers present, or the number of different atomspresent in a monolayer. It is understood that the number of monolayersmay be 9, 10, 11, 12, 13, etc. Typically the number of monolayers isbetween 1-8, more preferably between 1-6. The terminology where there isless than a whole number of monolayers is really a statistical average.One skilled in the art will appreciate that there is no such thing as afraction of a monolayer.

By “CdS/ZnS”, which is the terminology used to describe one example of a“graded shell”, it is meant to include any number of monolayers, andcompositionally this means that the first monolayer would be pure oralmost pure, i.e. greater than at least 90%, preferably at least 95%,and more preferably 99% CdS, with the next layer having a percentage ofZn in the CdS, and the next monolayer (if there is one) having a higherpercentage of Zn than the previous layer, and so on until the monolayerof ZnS is almost pure ZnS and practically no Cd, i.e. greater than least90%, preferably at least 95%, and more preferably 99% pure.

By “shaped nanorods” or “nanorods having a complex shape”, it is meantto include those nanorods having other than a rod shape, such as“branched” nanorods, especially those that are the subject of U.S.patent application Ser. No. 10/301,510, filed Nov. 20, 2002, entitled“Shaped Nanocrystal Particles and Methods for Making the Same”, thecontents of which are incorporated herein by reference in its entiretyfor all purposes. These include those having shapes of tetrapods, arrow,teardrop, and rods having one, two, three or more arms of varyinglength.

“Semiconductor nanorod” includes, for example, inorganic nanorodsbetween about 1 nm and about 1000 nm in diameter, preferably betweenabout 2 nm and about 50 nm, more preferably about 5 nm to about 20 nm(such as about 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20nm) that includes a “core” of one or more first semiconductor materials,and which may be surrounded by a “shell” of a second semiconductormaterial. A semiconductor nanorod core surrounded by a semiconductorshell is referred to as a “core/shell” semiconductor nanorod. Thesurrounding “shell” material will preferably have a bandgap greater thanthe bandgap of the core material and can be chosen so to have an atomicspacing close to that of the “core” substrate. The core and/or the shellcan be a semiconductor material including, but not limited to, those ofthe group II-VI (ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, MgTeand the like) and III-V (GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb,AlAs, AlP, AlSb, AlS, and the like) and IV (Ge, Si, Pb and the like)materials, and an alloy thereof, or a mixture thereof. It is alsounderstood that the term “semiconductor nanorod” may also include thosenanorods that have other than a semiconductor core, i.e. those with ametal core.

By “TOPO, TOP, TBP, HDA, HPA and TDPA” it is meant trioctylphosphineoxide, trioctylphosphine, tri-n-butylphosphine, hexadecylamine,hexylphosphonic acid and tetradecylphosphonic acid, respectfully.

By “method of using a nanorod barcode to identify an element” the term“element” does not strictly refer to “chemical element” as N, C, etc,but rather to any species of interest. Non limiting examples includebiological entities and compounds such as cells, proteins, nucleicacids, subcellular organelles and other subcellular components.“Element” also refers to other inorganic and organic compounds such aspolymers and catalysts.

It is understood that “semiconductor nanorod” may also refer to thatembodiment of this invention where there are more than one semiconductormaterial joined longitudinally to one another. This embodiment is alsoreferred to as “semiconductor nanorod barcode”.

A “graded shell” is meant to include semiconductor nanorods that have acore and a shell that comprises at least two monolayers of a material.Typically, many monolayers are present. In a preferred embodiment, theinvention contemplates a core material of CdSe, with a graded shell ofCdS/ZnS. In a preferred embodiment, this invention includes at least amonolayer of CdS overlying the CdSe core and at least a monolayer of ZnSoverlying the CdS layer, i.e. CdSe/CdS/ZnS. It is to be understood thatthere may be a small amount of S or other Group II, III, IV, V or VIelement (depending on the specific materials used in the shell) in theCdSe layer. The CdS layer may consist of monolayers, preferably between1-6, but not so limited. The monolayer closest to the CdSe layer wouldhave a minimal concentration of S and be almost pure CdS. The monolayerclosest to the outermost ZnS monolayer would be almost pure ZnS, with asmall amount of Cd. The monolayer of approximate intermediate distancebetween the outermost ZnS monolayer and the monolayer closest to thecore would have a compositional makeup of approximately 50/50% of ZnSand CdS. The instant invention contemplates that any Group II-VI orGroup III-V or Group IV semiconductor may be used as materials for thegraded shell. The same graded structure is present for differentsemiconductor materials.

By “optical pattern” it is meant the wavelength or wavelengths of light.

By “generating spectra” or “generating emission” it is meant the nanorodis capable of fluorescence or photoluminescence.

The term “barcode” as used herein refers to one or more sizes, sizedistributions, compositional makeup's, or combinations, of semiconductornanorods. The present invention contemplates that nanorod barcodes mayhave cores of other than semiconductor materials, those cores being ofmetals and metal compounds. Typical metals that are to be used as anon-semiconducting core include Co, Cu, Ni, Fe, Zn, all transitionmetals, oxides and nitrides thereof. Each size, size distribution and/orcomposition of semiconductor nanorods has a characteristic emissionspectrum, e.g., wavelength, intensity, FWHM, and/or fluorescentlifetime. In addition to the ability to tune the emission energy bycontrolling the structural properties, in particular size of theparticular semiconductor nanorod, the intensities of that particularemission observed at a specific wavelength are also capable of beingvaried, thus increasing the potential information density provided bythe semiconductor nanorod barcode system. For the purposes of thepresent invention, different intensities may be achieved by varying theconcentrations of the particular size semiconductor nanorod attached to,embedded within or associated with an item, compound or matter ofinterest. The “barcode” enables the determination of the location oridentity of a particular item, compound or matter of interest.

By “labeling” it is meant “linking”, “conjugating”, associating orbonding with the element of interest.

A semiconductor nanorod is “linked” or “conjugated” with, a specificmolecule or element when the semiconductor nanorod is chemically coupledto, or associated with the molecule. The terms indicate items that arephysically linked by, for example, ionic interactions, covalent chemicalbonds, physical forces such van der Waals or hydrophobic interactions,encapsulation, embedding, or the like. As an example without limitingthe scope of the invention, nanorods can be conjugated to molecules thatcan interact physically with biological compounds such as cells,proteins, nucleic acids, subcellular organelles and other subcellularcomponents. Also, nanorods can be associated with molecules that bindnonspecifically or sequence-specifically to nucleic acids (DNA RNA). Asexamples without limiting the scope of the invention, such moleculesinclude small molecules that bind to the minor groove of DNA.

The graded shell layer is particularly preferred because at the surfaceof the semiconductor nanorod, surface defects can result in traps forelectron or holes that degrade the electrical and optical properties ofthe semiconductor nanorod. An insulating layer at the surface of thesemiconductor nanorod provides an atomically abrupt jump in the chemicalnanorods, suitable materials for the layer should have good conductionand valence band offset with respect to the semiconductor nanorod. Thus,the conduction band is desirably higher and the valence band isdesirably lower than those of the semiconductor nanorod. Forsemiconductor nanorods that emit energy in the visible (e.g., CdS, CdSe,CdTe, ZnSe, ZnTe, GaP, GaAs) or near IR (e.g., InP, InAs, InSb, PbS,PbSe), a material that has a band gap energy in the ultraviolet legionsmay be used. Exemplary materials include ZnS, GaN, and magnesiumchalcogenides, e.g., MgS, MgSe, and MgTe. For semiconductor nanorodsthat emit in the near IR, materials having a band gap energy in thevisible, such as CdS or CdSe, may also be used. The shell layers mayinclude as many as eight or more monolayers of the semiconductormaterial.

The selection of the composition of the semiconductor nanorod, as wellas the size of the semiconductor nanorod, affects the characteristicspectral emission wavelength of the semiconductor nanorod. Thus, as oneof ordinary skill in the art will realize, a particular composition of asemiconductor nanorod as listed above will be selected based upon thespectral region desired. For example, semiconductor nanorods that emitenergy in the visible range include, but are not limited to CdS, CdSe,CdTe, ZnSe, ZnTe, GaP, and GaAs. Semiconductor nanorods that emit energyin the near IR range include, but are not limited to, InP, InAs, InSb,PbS, and PbSe. Finally, semiconductor nanorods that emit energy in theblue to near-ultraviolet include, but are not limited to ZnS and GaN.For any particular composition selected for the semiconductor nanorodscontemplated, it is possible to tune the emission to a desiredwavelength by controlling the size of the particular composition of thesemiconductor nanorod potential at the interface that eliminates energystates that can serve as traps for the electrons and holes. This resultsin higher efficiency in the luminescent process. Suitable materials forthe layer include semiconductors having a higher band gap energy thanthe semiconductor nanorod core. In addition to having a band gap energygreater than the semiconductor.

In one non-limiting embodiment, the instant invention contemplates thegrowth of a CdS/ZnS graded shell on CdSe semiconductor nanorods, in thepresence of a small amount of Cd precursor in trioctylphosphine oxide atlow temperature (160° C.). While not wishing to be bound by anyparticular theory or principle, it is believed that the CdS is morelikely to grow initially since its lattice mismatch with CdSe is lessthan that of ZnS. This layer of CdS mediates the growth of the morehighly strained ZnS, but the luminescence of the core/shell nanorods isnot increased very much. The shell growth is uniform and epitaxial,completely coating the CdSe core but may have defects present due to thelower growth temperature.

In one embodiment, the invention contemplates a photochemical annealingprocess, and the resulting nanorod has an increased luminescenceefficiency of from below 1% to up to 20-25%, while preserving theirsolubility in a wide range of solvents.

Current synthetic techniques for nanorods and core/shell nanorods relyon a convoluted interplay of kinetic and thermodynamic factors. Theinventors have determined that the large lattice mismatch between thecore and shell, for example, CdSe and ZnS was preventing thicker shellgrowth. To remedy this, small amounts of Cd(CH₃)₂ were added to thestock solution to facilitate growth of a CdS layer. CdS has a latticespacing between that of CdSe and ZnS, which decreases the overall strainin the system. In the stock solution used for this series ofexperiments, the Zn:Cd ratio was quite high (˜8:1), in order to promotethe growth of ZnS, using the CdS only as an intermediate between theCdSe and the ZnS. The Zn:S molar ratio was set higher than 1:1 to ensurea Zn rich surface. This allows the phosphine oxide, which specificallybinds to metal sites, to easily coordinate the surface of thenanocrystals. Upon gradual injection of the stock solution, a colorchange from dark red to brown was observed. The degree of color changewas dependent on the nanocrystal sample and on the amount of stocksolution added. In the following experiments, the amount of stocksolution injected ranged from 0.25 ml to 1.5 ml.

It is understood that the present invention contemplates graded shellson all sizes and shapes of nanorods. These include rod shaped nanorodsas well as nanorods having complex shapes.

The synthetic techniques described by the present invention allow forthe creation of nanorods with “tails”, which consist of a nanorod of onematerial and a tail of another material grown out of one or both ends ofthe initial nanorod, as well as repeats of this structure called“nanorod barcodes”. If the core is of a semiconductor material, the tailwill only grow out of only one end of the rod. This is true regardlessif the shape is rod-like, or other, such as tetrapods. If the core is ofa metal, the tail will grow out of both ends. Metals contemplated as acore include Co, Cu, Ni, Fe, Zn, all transition metals, oxides andnitrides thereof. Since inorganic nanocrystals possess optical,electrical, magnetic, catalytic, and mechanical properties that can bewidely tuned by variation of their size and shape, this inventionconnects these properties.

The present invention describes a systematic method for synthesizingnanorod barcodes based on conditions which favor growth of the 001 or00-1 faces of the hexagonal crystal structure. There is no limit to thenumber of sections in a nanorod barcode, but preferably the number issmall, for example 2-3. But nanorod barcodes with sections of 4, 5, 6, 7and more are possible. The sections of the nanorod barcode are bondedtogether with chemical bonds, either covalent or ionic bonds. Thematerials contemplated for the different sections of the nanorod barcodeare the same as those for the core material or the shell material,except that the core in a nanorod barcode is may be other than asemiconductor.

The growth of a tail of a semiconductor material, such as ZnS out of oneend of the nanorods also provides evidence of the intense strain presentin these particles. After 5-6 monolayers of shell are grown the straininduced by the lattice mismatch is too great to continue regular shellgrowth. To relieve this strain a tail grows out of one end of the rod,ZnS in this example. Since the tail consists solely of shell material,it only feels the strain for the few monolayers that connect it to thebody of the rod. The rest of the tail has the unstrained latticeparameters of ZnS. A similar situation occurs for the lumps that grow onthe other faces of the rod (the reason that a tail only grows out of oneside is due to the lack of inversion symmetry in the wurtzite structureand the higher energy of the 001 face relative to the others. This isobserved not only in the TEM but also in the XRD pattern of the thickshell samples (FIG. 4 e). In that pattern some small very broad peaks ofZnS are observed on top of the core/shell diffraction. These peaksresult from the tails and lumps that are diffracting as if they weresmall isolated domains of ZnS.

FIGS. 8-15 describe embodiments of nanorod barcodes in accordance withthe present invention. The cross sectional area of barcodes and alsosemiconductor nanorods is approximately hexagonal or circular. FIG. 10shows an embodiment where the cross sectional area of the tail or secondsegment is smaller than that of the first segment. FIG. 11 and FIG. 14describe embodiments where there are three segments, each havingdifferent cross sectional areas. One skilled in the art will appreciatethat by tailoring the growth parameters one can manipulate the crosssectional areas of the segments and thus tune the resulting emissions.FIG. 12 a is the core in one embodiment of the present invention andFIG. 12 b is the graded shell. FIG. 14 a shows the graded shell in oneembodiment of the present invention, and FIG. 14 b shows the core. FIG.15 a and FIG. 15 c describe semiconductor segments joined to a metalcore (FIG. 15 b).

A unique optical pattern or barcode can be detected when a nanorod isfunctionalized or unfuctionalized with a particular ligand, or bound toa particular target molecule or target molecules. When the nanorodencoded with an optical pattern or barcode is introduced into abiological or other system, it can be located and tracked using avariety of detection devices, usually optical detection devices. Forexample, a semiconductor nanorod barcode bound to a cell can be used totrack the movement of a cell in a biological fluid (e.g., blood, serum,lymph, semen, vaginal fluid).

The present invention contemplates two methods for synthesis ofnanorod-barcodes. Both involve synthesizing a nanorod of one materialand then growing an additional material out of one or both ends of theexisting rod. This can be done by injecting precursors of the secondmaterial in the same surfactant mixture as the first nanorods were grownin, thereby using these surfactants to control growth. Another method isto remove preform growth in a surfactant that does not promote shapecontrol, but to use the inherent lattice mismatch between two materialsto have strain induced rod growth out of one end of the initial nanorod.This procedure can be performed for any crystal with a hexagonalstructure whose lattice mismatch is not so large as to prevent anyepitaxial growth on the initial nanorod. This procedure is repeated sothat it is possible to add different materials to create a nanorodbarcode.

The graded semiconductor nanorods and barcodes of the present inventionare, optionally, surrounded by a “coat” of an organic capping agent.Thus the nanorod and/or barcode may be linked, conjugated, associatedwith a specific molecule, when the semiconductor nanorod is chemicallycoupled or associated with the specific molecule. The organic cappingagent may be any number of materials, but has an affinity for thesemiconductor nanorod surface. In general, the capping agent can be anisolated organic molecule, a polymer (or a monomer for a polymerizationreaction), an inorganic complex, and an extended crystalline structure.The coat is used to convey solubility, e.g., the ability to disperse acoated semiconductor nanorod homogeneously into a chosen solvent,functionality, binding properties, or the like. In addition, the coatcan be used to tailor the optical properties of the semiconductornanorod.

EXAMPLES

The examples provided herein are provided as examples and notlimitations, wherein a number of modifications of the exemplifiedprocess are contemplated and within the scope of the present invention.

I. Materials

Dimethylcadmium (Cd(CH₃)₂, 97%) and tri-n-butylphosphine (C₁₂H₂₇P orTBP, 99%) were purchased from Strem. Cd(CH₃)₂ was vacuum transferred andstored at −35° C. under argon. Selenium (Se) (99.999%),tri-n-octylphosphine oxide (C₂₄H₅₁OP or TOPO, 99%), diethylzinc(C₄H₁₀Zn, or Et₂Zn, 1.0 M solution in heptane) andhexamethyldisilathiane (C₆H₁₈Si₂S or (TMS)₂S) were purchased fromAldrich. Hexylphosphonic acid (C₆H₁₅O₃P or HPA 99%) was purchased fromOrganometallics Inc., and tetradecylphosphonic acid (C₁₄H₃₁O₃P or TDPA,98%) was purchased from Alfa. All solvents used were anhydrous,purchased from Aldrich and used without any further purification.

II. Stock Solutions. Stock solutions were prepared in a dry box under Arand then placed in a refrigerator at −20° C. For the synthesis of CdSenanorods, we prepared the solution for each precursor separately. Forthe Se precursor, selenium powder was dissolved in TBP (concentration ofSe 7.79% by weight). For the Cd precursor, Cd(CH₃)₂ was dissolved in TBP(concentration of Cd 32.29% by weight). The stock solution for the ZnSshell was prepared by dissolving 152 mg of (TMS)₂S and 0.63 g of theEt₂Zn solution in 4.1 g of TBP. In this solution the Zn:S molar ratio is1:1. The stock solution for the CdS/ZnS graded shell was prepared bymixing 0.5 g of the Et₂Zn solution, 37 milligrams of a solution ofCd(CH₃)₂ in TBP (32.29% by weight) and 76 mg of (TMS)₂S. The resultingsolution was then diluted in 2.05 g of TBP. In this solution the Zn:Cd:Smolar ratio is 1:0.12:0.63.

III. Synthesis of CdSe Rods. All manipulations were performed usingstandard air-free techniques, unless otherwise stated. In a typicalsynthesis, a mixture of HPA, TDPA and TOPO²⁰ was degassed at 120° C. for1 hour in a 50 ml 3-neck flask connected to a Liebig condenser, afterwhich 0.5 g of the Cd precursor solution was added drop wise. Theresulting mixture was then heated to 360° C. and 2.5 g of the Seprecursor solution was quickly injected. After injection, thetemperature dropped to 290° C. and was maintained at this levelthroughout the synthesis. When desired, the synthesis was stopped byremoving the heating mantle and by rapidly cooling the flask. In thepresent series of experiments, we prepared 4 samples of CdSe rods ofdifferent lengths and aspect ratios by varying the relativeconcentration of TOPO:HPA:TDPA and the growth time. The details arereported in Table 1.

After cooling the solution to 50° C., 4.0 ml of methanol was added toprecipitate the rods from the solution. This suspension was thentransferred to a dry box, where it was centrifuged and the precipitatewas washed three times with methanol. The final precipitate was thendried under Ar and stored in the dry box. Due to the high degree ofuniformity of the rods that this synthesis procedure yields, no furthersize selective precipitation was carried out on any samples.

IV. Epitaxial Growth of CdS/ZnS Graded Shell. 5 grams of TOPO was placedinto a 50-ml 3-neck flask, pumped under vacuum at 120° C. for 20 minutesand then cooled to 60° C. Ten mg of dry nanorods were dissolved in 2.0ml of chloroform. This solution was removed from the glove-box andinjected into the TOPO solution at 60° C. The chloroform was removed bypumping the mixture under vacuum for 20 minutes. The temperature of themixture was raised to 160° C. Depending on the desired thickness of theshell, a given amount (see Table 2) of the CdS/ZnS stock solution wasloaded into a syringe and injected dropwise into the flask. A typicalinjection rate for this series of experiments was around 0.1 ml/min.After the injection was completed, the solution was kept at 160° C. for10 minutes. During this time the shell growth was completed. Thetemperature in the flask was then lowered to 40° C. and 3.0 ml ofoctanol was added to quench the unreacted precursors. The resultingsolution was immediately transferred under Ar into the glove box andstored in the dark.

V. Precipitation and Re-Dissolution of Core/Shell Rods

The solution of nanocrystals in TOPO/TBP/octanol was stable, opticallyclear, and no precipitate was observed even several months after thesynthesis. Addition of methanol to this solution caused theprecipitation of the nanocrystals, which could then be easilyredissolved in solvents such as chloroform, toluene, or tetrahydrofuran.There were a few cases in which the core/shells did not redissolve. Toavoid this problem, we found it was very effective to add a small amount(1 mg/ml) of a phosphonic acid, such as hexylphosphonic (HPA) acid, orof an amine such as hexadecylamine (HDA). In this case, after methanolwas added, the solution immediately turned turbid and the collectedprecipitate could then be readily redissolved. Solubility problems werealso encountered when a precipitate (obtained without the addition ofHPA or HDA) was washed several times with methanol. Here the addition ofHPA or HDA to the solvent caused the immediate redissolution of theparticles. Henceforth we will call these samples ‘HPA-capped’ and‘HDA-capped’ nanorods, respectively. This is to distinguish them fromsamples of nanorods precipitated and redissolved without the assistanceof additional surfactants, which will be called ‘TOPO-capped’nanorods^(45,46). In addition, we will call ‘raw nanorods’ the samplesobtained by simply diluting in chloroform the original solution ofnanocrystals in TOPO/TBP/octanol, without any precipitation orredissolution procedures.

VI. Photochemical Shell Annealing. Laser irradiation experiments tophotochemically anneal the shells were carried out by exposing a 1 cmpath-length quartz cuvette filled with a diluted solution of CdSenanorods or CdSe/CdS/ZnS core/shell nanorods to a continuous Ar⁺ laser(Lexel 95 ion laser, Lexel Laser, Inc.). The power of the laser wastuned between 50 and 120 mW, depending on the particular experiment. The457.9 nm and the 514.5 nm line were alternatively used as excitationlines. The laser spot on the sample had a diameter of approximately 1cm. The number of nanoparticles in the cuvette was estimated byevaluating the average weight of a single nanorod, and the total amountof CdSe in the solution. By measuring the laser power absorbed by thenanocrystal solution it is possible to calculate the average number ofphotons absorbed by each particle per second of exposure to the laserlight.

VII. Preparation of a Two Segment Nanorod Barcode. Described is thegrowth of ZnS tail on CdSe nanorod. 5 grams of TOPO was placed into a50-ml 3-neck flask, pumped under vacuum at 120° C. for 20 minutes andthen cooled to 70° C. 20 mg of dry nanorods were dissolved in 2.0 ml ofchloroform. This solution was removed from the glove-box and injectedinto the TOPO solution at 70° C. The chloroform was removed by pumpingthe mixture under vacuum for 20 minutes. The temperature of the mixturewas raised to 160° C. After 1 hour at 160 C, 0.5 mL of ZnS stocksolution was loaded into a syringe and injected dropwise into the flask.A typical injection rate for this series of experiments was around 0.1ml/min. After injection this process was repeated until a total of 2.5mL ZnS stock solution was injected into the flask (in another example wealso used 4.0 mL of ZnS stock solution). After the injection wascompleted, the solution was kept at 160° C. for 10 minutes. During thistime the tail growth was completed. The temperature in the flask wasthen lowered to 40° C. and 3.0 ml of methanol was added to quench theunreacted precursors. The resulting solution was immediately transferredunder Ar into the glove box and stored in the dark.

VIII. Preparation of a Two Segment Nanorod Barcode With a Graded Shell.Described is the growth of ZnS tail on CdSe nanorod with graded CdS/ZnSshell. 5 grams of TOPO was placed into a 50-ml 3-neck flask, pumpedunder vacuum at 120° C. for 20 minutes and then cooled to 60° C. Ten mgof dry nanorods were dissolved in 2.0 ml of chloroform. This solutionwas removed from the glove-box and injected into the TOPO solution at60° C. The chloroform was removed by pumping the mixture under vacuumfor 20 minutes. The temperature of the mixture was raised to 160° C.After 1 hour at 160 C, 1.5 mL of the CdS/ZnS stock solution was loadedinto a syringe and injected dropwise into the flask. A typical injectionrate for this series of experiments was around 0.1 ml/min. After theinjection was completed, the solution was kept at 160° C. for 10minutes. During this time the shell growth was completed. Thetemperature in the flask was then lowered to 40° C. and 3.0 ml ofoctanol was added to quench the unreacted precursors. The resultingsolution was immediately transferred under Ar into the glove box andstored in the dark. One skilled in the art will appreciate thatadditional segments may be added by repeating the growth process.

IX. Preparation of a Two Segment Nanorod Barcode With a Metal Core.Growth of ZnS tail on Co nanorod. 5 grams of TOPO may be placed into a50-ml 3-neck flask, pumped under vacuum at 120° C. for 20 minutes andthen cooled to 60° C. 20 mg of dry Co nanorods are dissolved in 2.0 mlof chloroform. This solution may then be removed from the glove-box andinjected into the TOPO solution at 60° C. The chloroform may then beremoved by pumping the mixture under vacuum for 20 minutes. Thetemperature of the mixture can be raised to 160° C. After 1 hour at 160C, 1.5 mL of the ZnS stock solution should be loaded into a syringe andinjected dropwise into the flask. A typical injection rate for thisseries of experiments is around 0.1 ml/min. After the injection wascompleted, the solution can be kept at 160° C. for 10 minutes. Duringthis time the tail growth should be completed. The temperature in theflask will then be lowered to 40° C. and 3.0 ml of methanol is added toquench the unreacted precursors. The resulting solution is immediatelytransferred under Ar into the glove box and stored in the dark.

X. Characterization of Samples. All sampling procedures for the opticalcharacterization of the samples were carried out under Ar unlessotherwise stated. In the case of CdSe nanorod cores, a small amount ofsample (˜0.2 ml) was removed via syringe from the flask before the shellgrowth. In the case of core-shell nanorods, a small amount of the finalsolution stored in the dry box (˜0.2 ml) was used: depending on theparticular experiment, this solution was processed according to one ofthe procedures described in the previous section. The sample was dilutedto an optical density of between 0.1 and 0.25 by addition of anhydrouschloroform in a glove box.

A. UV-Vis Absorption Spectroscopy. Absorption spectra were measuredusing a Hewlett Packard 8453 UV-visible diode array spectrometerequipped with a deuterium lamp having a resolution of 1.0 nm.

B. Photoluminescence Spectroscopy. Photoluminescence (PL) spectra wererecorded on a Spex 1681 0.22 m/0.34 m spectrometer. PL quantumefficiencies of the nanorods in chloroform were calculated by comparingtheir integrated emission to that of a solution of Rhodamine 6G inmethanol. Optical densities of all solutions were adjusted to between0.1 and 0.25 at the excitation wavelength to avoid re-absorptioneffects. The excitation wavelength used for all measurements was 480 nm.Emission spectra were corrected for the wavelength dependent response ofthe photomultiplier tube and for the refractive indexes of methanol andchloroform.

C. Transmission Electron Microscopy. Nanocrystal size, morphology andstructure were measured via TEM. At the National Center for ElectronMicroscopy at Lawrence Berkeley National Laboratory, a Topcon EM002Belectron microscope was used. The microscope was operated at anaccelerating voltage of 120 kV. At the UC-Berkeley Electron MicroscopeLab, a FEI Tecnai 12 was used with an operating voltage of 120 kV.

Nanocrystals were deposited from dilute solution onto a 3-4 nm thickfilm of amorphous carbon supported by 400 mesh copper grids (purchasedfrom Ted Pella). One drop of nanocrystal solution in chloroform wasdeposited onto the grid and evaporated.

Structural determination was accomplished using high resolution TEM(HRTEM) at 550,000 times magnification. Average sizes and morphologieswere measured at 140,000 times magnification, calibrated using knowncrystal lattice spacings. Average lengths and shape distributions weredetermined by counting at least 200 nanocrystals per sample forstatistical purposes.

D. EDX. Energy dispersive X-ray Spectroscopy (EDX) was performed using aPhilips CM200/FEG at the National Center for Electron Microscopy atLawrence Berkeley National Laboratory. This microscope was operated atan accelerating voltage of 200 kV using an Oxford Model 6767 energydispersive X-ray detector with an energy resolution of 1.36 eV for Mn Kαradiation. Between twenty and one hundred nanorods were used per scanand at least 10 scans were taken per sample. The average scan time wasbetween 20 and 40 minutes. For composition determination, scans times ofthe same length were used, and EDX scans were normalized to the Se Kαline.

E. Powder X-ray Diffraction. Powder X-ray diffraction was performed on aBruker-AXS D8 general area detector diffraction system (GADDS), using CoKα radiation (1.79026 Å). Two-dimensional patterns were angle integratedto obtain the patterns displayed. The instrument resolution is 0.07° in2θ and the accumulation time for each frame of each sample was 20minutes. Three frames were taken per sample, centered at 2θ angles of25°, 40°, and 55°, and at Ω angles of 12.5°, 20°, and 27.5°respectively. XRD samples were prepared by evaporating a concentratednanocrystal solution on a quartz plate. Prior to the measurements, thesamples were washed with methanol to remove excess organic material andthen dried. All Peaks were fit using commercial software (PeakFit™ v4)utilizing a Gaussian*Lorentzian peak shape.

II. Structural Characterization. The invention contemplates that thethickness of the graded shell can be controlled by the amount ofprecursor combined; preferably injection is used. Several techniques areused to monitor and characterize the shell's thickness, structure,crystallinity, and composition. FIG. 1 shows low resolution TEM imagesof a CdSe core nanorod (3.3×22.8 nm) (a), the same cores with a thinshell (b), a medium shell (c), and a thick shell (d). In these imagesone can see the increase in diameter of the nanorods from 3.3 nm in thecores to 4.4 nm in the thin shell sample, 6.0 nm in the medium shellsample, 7.3 nm in the thick shell sample. This corresponds to growingroughly 2, 4.5, and 6.5 monolayers of CdS/ZnS shell for the thin,medium, and thick shell samples respectively. The shell is observed tobe very regular and conform to the shape of the underlying CdSe core aslong as the shell is less than 5-6 monolayers thick. This is true notonly for the medium length rods shown in FIG. 1, but is observed to beindependent of aspect ratio as seen in FIG. 2. The shell growth seems toimprove the overall regularity of the rods in that they seem to bestraighter than the cores themselves (the irregularities in the coresare caused by zinc blende stacking faults in them).

FIG 1 d, shows a tail is observed to grow straight out of one end of therods and the overall surface of the rods becomes rough. It is importantto note that separate nuclei of CdS or ZnS particles were not observedvia TEM in any of our samples. Thus, only shells are grown, and notseparate nanorods or nanocrystals.

In addition to the above TEM, HRTEM and XRD were used to determine thestructure and crystallinity of the core/shell structures. In FIG. 3, theHRTEM images of the medium length nanorod cores (a), the same cores witha thin shell (b), medium shell (c), and thick shell (d) of CdS/ZnS areshown. The lattice fringes are continuous through both the core andshell implying epitaxial growth of the shell. There were no obviousstacking faults or defects observed at the interface of core and shell.In addition, the core and shell both have a wurtzite structure and theyincrease in both diameter and length as the shell thickness increases,although the length increases slightly faster than the diameter. Thethickest shell samples, where the growth of a ZnS tail is observed, aremore difficult to image in the HRTEM. At low magnifications, with alarge beam spot size, the thin tails can be clearly observed (FIG. 1 d),but when the spot size is decreased at higher magnifications (˜550 kX)the tails are damaged by the beam faster than an image can be taken.

Powder X-ray diffraction patterns of the same samples are shown in FIG.4. The bulk pattern of CdSe (FIG. 4 a) matches exactly that of the CdSenanorod cores with the exception of the relative intensities of thepeaks. The sharp 002 peak (29.6 °2θ) results from the extendedcrystalline domain along the c-axis of the wurtzite lattice. As theshell is grown and its thickness increases, the diffraction peaks shifttowards smaller d-spacings (larger 2θ). This means that the growth ofthe CdS/ZnS shell is compressing the lattice planes in the CdSe core andthat the compression increases as a function of shell thickness. Since aCdSe rod is an anisotropic system (in both crystal structure and shape),we should expect this compression to have a different effect on thevarious families of planes of the crystal. The shifts of differentdiffraction peaks in samples with increasingly thicker shells are shownin Table 3.

The diffraction patterns from the thin and medium thickness shellsamples show that, apart from the aforementioned shifts, the peak widthsare almost unchanged as compared to the core spectrum and that noadditional peaks are present. In the thickest sample though, the peakshave broadened significantly and there may be small broad diffractionpeaks that overlap with the ZnS bulk peaks (FIG. 4 f). Once tails havebegun growing on the rods, they diffract as if they were small domainsof isolated ZnS. Due to Debye-Scherrer broadening, they are observed asvery broad peaks with low intensity.

To determine the shell composition EDX spectroscopy was used. Again, thesame four samples characterized by TEM, HRTEM, and XRD were analyzed.All of the spectra were normalized to the Se Kα line since the amount ofSe in the cores and core/shells remains constant. The lines used areshown in FIG. 5. It is clear that there is no Zn or S in the CdSe corenanorods. As the shell thickness increases the amount of these elementspresent in the shell also increases. The Cu line is due to scattering inthe microscope off the Cu TEM grid and was subtracted when determiningthe area of the Zn Kα line. Since we are also adding Cd, the amount ofCd also increases with increasing shell thickness as seen in the Cd Lαand Lβ spectra. Combining the EDX data with the sizes collected via TEMthe composition of the shell was determined as a function of shellthickness. CdS makes up 35% of the shell in the thin shell sample, 22%of the shell in the medium shell sample, and 22% of the shell in thethick shell sample. Since the thin shell sample has slightly less thantwo monolayers of shell grown on it, this corresponds to about ⅔ of amonolayer of CdS, the remainder being ZnS. As the thickness increases,the CdS continues to grow, but the ratio of Zn:Cd increases as seen inTable 2.

III. Optical Characterization and Photochemical Annealing

The significant red shift in the absorption and emission spectra fromcore/shell samples also confirms that a relevant percentage of the shellis composed of CdS. Given the small energy difference between the bottomof conduction bands in CdSe and in CdS (0.2 eV in the bulk limit), thephoto-generated electrons in colloidal CdSe/CdS dots can easily tunnelfrom the CdSe cores into the CdS shell (the core electrons have toovercome a potential step of 0.55 eV). When the thickness of the CdSshell increases, the absorption and luminescence spectra of CdSe/CdSdots shift to lower energies, since the confinement energy for theelectrons is lower. This effect is less remarkable in CdSe/ZnS dots,where the potential barrier for both carriers to tunnel from the CdSecores into the ZnS shell is approximately 0.9 eV but a red shift isstill expected.

FIG. 6 shows the absorption spectra (solid line) of CdSe core nanorods,and the same cores with two different thickness shells. The solutionswere ‘raw’, as described in the paragraph V of the experimental section.In the core-shell samples there is a progressive red shift in thespectral features, with respect to the starting CdSe cores, as thethickness of the shell increases. This suggests that CdS is included inthe shell, since a less remarkable shift would be expected from acore-shell system with only ZnS in the shell. The cores had aluminescence quantum yield (QY) lower than 1%, the thin shell sample hada QY of 4%, and the QY of the medium shell sample was again less than1%.

FIG. 6 also shows the emission spectra of the three samples (dottedlines) after exposure to laser light for 8 hours (The exposure time ofthese samples was 20 hours on average. The exciting wavelength was 514.5nm, the laser power was approximately 80 mW and the optical density ofall solutions was 0.2 at 480 nm. The QY from the thin shell sampleincreased to 7%, whereas the QY from the medium shell sample increasedto 16%. Further exposure to laser light did not affect the QY from thecore/shell samples. On the other hand, the same laser treatment on theCdSe cores did not significantly increase their QY. In fact theirluminescence either remained constant or decreased during laserirradiation. In all samples, the laser treatment did not cause anyrelevant spectral shift in absorption or luminescence, or any change inthe optical density of the nanocrystal solutions: this rules out anypossibility that the shell grew or shrank during laser illumination. Inaddition, TEM, HRTEM, and XRD were carried out on samples before andafter illumination. No noticeable shape change was observed.

An increase in luminescence was observed in all core/shell samples thatwere photoannealed. Solutions of non-annealed core/shell rods kept underdark had low QY, even several weeks after the synthesis. Afterphotoannealing for a few hours, their QY increased without any spectralshift. FIG. 7 a shows the measured QY for a CdSe core sample andcore/shell as a function of the average number of photons absorbed perparticle. The core/shell sample was raw⁴⁸. The nanorods in the CdSe coresample had a length of 21 nm and a diameter of 3.3 nm. In the core/shellsample, the ZnS shell was 2.5 monolayers thick. The core/shell sampleunderwent photochemical annealing as its QY started from 3% andsaturated at 17%, whereas the QY from the core sample remains less than1%.

To check the influence of the post-synthesis treatment on thephotochemical annealing, the above experiments were repeated on the samecore/shell sample, but cleaned according to the procedures described inthe paragraph V of the experimental section. Three additional sampleswere then prepared from the original TOPO/TBP/octanol solution:TOPO-capped, HPA-capped, and HDA-capped core/shell. All these samplesdisplayed quantum yield enhancement following photochemical annealing,although the HDA-capped sample reached the highest QY (it started at 4%and increased to 21%), the HPA-capped sample experienced the lowestincrease in QY (from 3% to 8%), and the TOPO-capped sample went from 2%to 11%. Also checked was the effect of laser exposure on the CdSenanorod cores capped with HDA. After overnight laser exposure, theHDA-coated CdSe nanorod cores experienced a degradation in their QY(from 4% to 2%).

All laser exposure experiments were repeated using the 457.9 nm laserline excitation. For the core/shell rods the QY versus photons absorbedper particle showed the same behavior as in the previous experiments,saturating after the same number of absorbed photons, whereas CdSe coresagain experienced either no change or a decrease in QY. This indicatesthat the photoannealing process is not dependent on the excitationenergy, just that the energy is absorbed by the nanocrystal. In allcases the QY of the photoannealed core/shells was unexpectedlysurprisingly much higher than the initial, pre-annealing value. Moreimportantly, this process is irreversible: all cleaned samples ofphotoactivated core/shell nanocrystals, left in the dark under Ar formonths, did not increase or decrease their QY by significant amounts(less than 5% variation was considered to be within the experimentalerror). In addition there is no change in the fluorescence peak shape orpeak maximum over this period of months. This is true for all of thesurfactant-exchanged core/shells and core/shells that were washed withmethanol at least once to remove any unreacted precursors, beforephotoannealing. The only sample that showed any change over time was theraw core/shell nanorods. After photoannealing, this sample underwent avery slow degradation process, most likely due to the presence ofreactive species in the solution. Given the lower stability of rawcore/shell nanorods, all quantitative studies were carried out onsurfactant exchanged or cleaned core/shell samples.

IV. Photostability of the Core/Shell Rods

The solutions of photoactivated core/shell nanorods (obtained asdescribed in the previous section) were opened to air and exposed againto laser light to check their stability against photooxidation. Duringthese experiments, every nanocrystal in each sample absorbedapproximately 3.5*10⁹ photons over 8 hours⁵⁰. The HDA-capped sampleshowed the highest stability, with no change in QY and no blue shift inthe luminescence peak. The HPA-capped and the TOPO-capped samples bothexperienced a blue shift of approximately 10 meV (4 nm) in their PLpeak. The QY from the TOPO-capped sample slightly decreased (from 11% to10%), whereas the PL from the HPA capped increased (from 8% to 14%). Thecorresponding cores oxidized more under the same conditions.

The growth of highly luminescent core/shell nanorods elucidates threebasic concepts in interfacial growth: when both ZnS and CdS precursorsare added, CdS preferentially grows first to reduce the interfacialenergy; photoannealing permanently changes the core/shell nanorods,implying a structural reorganization; and growing shells on nanorodsallows for the study of strain in a system that is intermediary betweena “0D” nanocrystal core and a “2D” bulk surface.

Interfacial segregation is required for shell growth in this system, andyet CdS grows first on the CdSe core. Although the ratio of Zn:Cdinjected is ˜8:1, the ratio of ZnS:CdS in the first two monolayers ofshell is only 2:1. As the shell thickness increases this ratio goes from2:1 to 4:1 to ˜4.5:1 and levels out as the shell thickness increases.The concentration of Zn in the precursor solution cannot compensate forthe larger ZnS lattice mismatch, so initially Cd is more likely to stickto the surface of the CdSe cores. The farther away from the core (or thethicker the shell), the more the ratio depends solely on theconcentration. When no Cd is added, the strain is so great that eitherno shell grows at all, or a tail of ZnS grows out one end of thenanorods.

This interfacial segregation could also be partially driven by the lowersolubility of CdS with respect to ZnS in the surfactant used to grow theshell (TOPO). This concept, of selective precipitation has been used togrow CdS/HgS/CdS quantum dot/quantum wells in aqueous solvents due tothe large difference in solubility products of Cd²⁺ and Hg²⁺, seeEychmuller, A. et al. Chem. Phys. Lett 1993, 208, 59-62. the contents ofwhich are hereby incorporated herein by reference in its entirety forall purposes. While not wishing to be bound by any particular theory orprinciple, it is known that Cd atoms form less stable complexes withTOPO than Zn atoms do, and this may influence the order in which theatoms add to the core nanorod surface. If the shell growth were purelydue to solubility differences, then it might be possible to grow a pureZnS shell on CdSe core nanorods. However, this is not observed becausethe strain between CdSe and ZnS is too large. Therefore, while notwishing to be bound by any particular theory or principle, the inventorsbelieve that the main mechanism responsible for the formation of gradedshells in our system is strain-induced interfacial segregation.

In a typical synthesis of core/shell dots or nanocrystals, such as theCdSe/ZnS or the CdSe/CdS system, as the thickness of the shellincreases, the luminescence QY first increases, and then declines. Thistrend is believed to be a consequence of increased strain in the shell.As long as the strain can be tolerated, the epilayer passivates theinterface trap states and does not create additional mid-gap states.Once past a certain shell thickness, the strain is released through theformation of dislocations in the shell. Dislocations act asnon-radiative recombination centers and lower the QY. In the presentinvention, the inventors noticed the same trend under normal conditions.However, the samples were irradiated with laser light, the QYs increasedsignificantly: even samples with a thick irregular shell had a QYgreater than 10% after this process.

The low luminescence observed in samples before laser irradiationindicates that a significant amount of non-radiative recombinationcenters are present throughout the shell. There is a permanent increasein QY of our core/shell nanorods, after laser irradiation, suggestingthat the laser induces a structural reorganization in the shell. Thelaser power was kept low enough that the temperature of the solutionremained constant. In addition, experiments were performed wheresolutions of core/shell nanorods were externally heated to 160° C. andthe photoluminescence was monitored. Even after hours at this elevatedtemperature there was no increase in QY. This implies that aphotochemical process is responsible for annealing the core/shells.Thermal annealing cannot be performed above the growth temperature of160° C. since other processes such as Ostwald ripening or dissolution ofthe particles can occur.

Structural changes that lead to a permanent change in the luminescenceQY of semiconductor dots or films are known to occur under high-powerlaser excitation, although there are known cases where these changesoccur under photoexcitation. For instance, disordered ZnS:Mn filmsshowed enhanced luminescence from Mn²⁺ ions when irradiated byultraviolet laser with energy pulses well below the conventionalannealing threshold. This was explained by the low energy of formationand diffusion of defects in disordered semiconductors. Although incore/shell rods the evidence brought by TEM and XRD results can rule outa highly disordered shell, a certain number of defects are likelypresent at the highly strained interface. There is the possibility thatchemical bonds at the interface can rearrange, or that defects candiffuse to the outer surface through a photochemically activatedprocess. This is possible because the shell is only few nanometersthick. In addition, laser irradiation can induce surfacereconstructions, which would decrease the number of surface trap states.

TEM, HRTEM, and XRD were performed on samples before and afterphotoannealing. There were no shape or structural changes observed usingany of these methods. This is not surprising, however, considering thefact that all of these techniques rely on diffraction from planes ofatoms, and are not sensitive to the positions of individual atoms. Asborne out by simulations, see Wickham, J. N. et al. Phys. Rev. Lett.2000, 84, 923-926, (the contents of which are hereby incorporated hereinby reference in its entirety for all purposes, these techniques are notsensitive to the surface atoms or the individual atoms at the interfaceof the core/shell nanocrystals. Any structural changes occurring at thesurface or interface would therefore not be observed.

The surfactant dependence of the core/shell QY can be understood byconsidering that in this system the carriers are not completelylocalized in the core and can sample the outer surface of the rod. Thisexplains why some surfactants (long chain alkyl amines) increase theluminescence from core/shell nanocrystals by neutralizing surface trapstates, whereas other molecules (such as pyridine) decrease it. Bulkysurfactants, such as TOPO, are not able to passivate all the metal siteson the surface and are therefore less efficient than alkyl amines. Amore uniform surface coordination, such as the one offered by alkylamines, also imparts a higher stability against photooxidation. Althoughin our case, the addition of different surfactants leads to differentinitial and final QYs, the annealing process followed the same behavioras a function of incident photons. If the increase in QY were the resultof the surfactant reacting with the surface of the shell then we wouldnot observe the same behavior by different surfactants with differentfunctional groups.

All of this evidence indicates that the photoannealing leads to apermanent change in the core/shell nanorods. Such a permanent change notonly rules out photobrightening or oxidation as the cause of theincreased QY, but also supports the theory that a structuralrearrangement has occurred. Since there are no obvious changes observedin the HRTEM and XRD of the annealed sample, but the changes arepermanent, the annealing is most likely only affecting the core/shellinterface or the surface of the shell.

Core/shell nanorods provide a unique system for the study of strain inshell growth. Unlike in 2D epitaxial growth, the substrate (in this casethe nanorod) is not fixed, so the lattice planes can actually becompressed by growth of the shell material. In addition, since this is a1D system, some of the crystal faces behave more like those in a 2Dsystem, while other faces behave more like the 0D surfaces of a highlycurved spherical nanocrystal. The growth of shells with such a highlattice mismatch accentuates the induced strain and XRD provides a meansto observe the strain induced by the CdS/ZnS shell. All of thediffraction peaks shift to lower d-spacings (higher 2θ) as a function ofshell thickness. Upon shell growth, the 002 peak shifts the most of anyof the diffraction peaks (in all samples). Each plane in the (002)family of planes extends for only 3-4 nanometers. On the other hand, the100 peak is generated by planes that are parallel to the c-axis. Each ofthese planes extends along the whole length of the rods, which is atleast 3 to 8 times larger than the (002) planes in the nanorods. Thispeak shifts less than any other diffraction peak (in all samples) as afunction of shell thickness. All of the other diffraction peaks shift byamounts that are intermediate between the shifts in the 002 and the 100peaks. This implies that the planes extending along the diameter of therod (or having a significant component along the diameter) are morecompressed than planes extending along the length of the rod. Thisobservation can be understood if one considers that, unlike traditionalmethods of shell growth where the substrate is fixed (bulk), thesubstrate in this case is thin enough that the shell can actuallycompress the lattice planes of the core. This compression of planes ismore pronounced at their edges, near the core/shell interface. Sinceplanes made up of very few atoms, such as the ones along the diameter ofthe rods, are more affected by this perturbation, their averaged-spacing will change more than the extended planes with many atoms.These larger planes, such as the (100) planes, which extend along thelength of the nanorod, may only be compressed at their ends, but notthroughout the entire crystal length.

In addition, the intensity of the 002 peak decreases relative to theother peaks in the sample. This is not what one might expect since TEManalysis shows that the average length of the rods is increasing withshell thickness. The increase in length should make the 002 peaknarrower and more intense as the domain size is increasing. Once we takeinto account the strain, however, these results make sense. As thecompression of the 002 planes increases with shell thickness, this willcause a broader distribution of observed domain sizes, thereby spreadingout the 002 peak and decreasing its intensity, while the otherd-spacings, and therefore peaks are not affected as significantly.

TABLE 1 CdSe core nanorod growth conditions and corresponding averagesizes. Rod length Rod diameter HPA TDPA TOPO Reaction time (nm) (nm) (g)(g) (g) (min) 18 5.0 0.04 0.456 3.50 5 23 3.3 0.13 0.34 3.55 2 21 3.30.08 0.39 3.53 4 36 4.5 0.13 0.34 3.55 5

TABLE 2 Core/shell nanorod growth conditions, final average sizes,number of shell monolayers and composition of the shell. # of mL of RodRod Shell CdS Zn:Cd Stock length diameter Mono- in in Sample Solution(nm) (nm) layers Shell Shell Core 0 22.8 3.3 0  0% NA Thin Shell 0.624.2 4.4 2 35% 2:1 Medium Shell 0.75 27.0 6.0 4.5 22% 4:1 Thick Shell1.5 29.8* 7.3 6.5 22% 4.5:1  *The given length is the average “body”length and does not include the tail if present

TABLE 3 XRD peak changes as a function of shell thickness. 002 100 002Peak d-spacing 100 Peak d-spacing Sample Position change Position changeCore 29.6 0.0% 28.1 0.0% Thin Shell 30.05 1.5% 28.4 0.9% Medium 30.352.4% 28.6 1.5% Shell Thick Shell 30.75 3.7% 28.75 2.0%

The terms and expressions which have been employed herein are used asterms of description and not of limitation, and there is no intention inthe use of such terms and expressions of excluding equivalents of thefeatures shown and described, or portions thereof, it being recognizedthat various modifications are possible within the scope of theinvention claimed. Moreover, any one or more features of any embodimentof the invention may be combined with any one or more other features ofany other embodiment of the invention, without departing from the scopeof the invention.

All patents, patent applications, and publications mentioned above areherein incorporated by reference in their entirety for all purposes.None of the patents, patent applications, and publications mentionedabove are admitted to be prior art.

1. A method of growing a graded core/shell semiconductor nanorod,comprising: providing a semiconductor nanorod core, combining the corewith at least one surfactant to form a surfactant/core mixture; heatingthe surfactant/core mixture, wherein the mixture is heated to atemperature between 100-360° C.; and combining the mixture with asolution, wherein said solution comprises semiconductor precursors in amolar ratio sufficient to cause the growth of a graded semiconductorshell on the core, and wherein the graded semiconductor shell comprise afirst compound and a second compound, wherein at least one element ofthe first compound and the second compound is in a greater amount closerto the core than away from the core.
 2. The method of growing a gradedcore/shell semiconductor nanorod of claim 1, wherein: the semiconductornanorod core comprises a semiconductor material selected from the groupconsisting of Group II-VI, Group III-V and Group IV semiconductors. 3.The method of growing a graded core/shell semiconductor nanorod of claim1, wherein: the core is rod shaped.
 4. The method of growing a gradedcore/shell semiconductor nanorod of claim 1, wherein: the core comprisesCdSe.
 5. The method of growing a graded core/shell semiconductor nanorodof claim 1, wherein: the mixture is heated to a temperature of 160° C.6. The method of growing a graded core/shell semiconductor nanorod ofclaim 1, wherein: only one surfactant is combined with the core.
 7. Themethod of growing a graded core/shell semiconductor nanorod of claim 1,wherein: the surfactant is chosen from the group consisting of TOPO,TBP, HDA, HPA and TDPA.
 8. The method of growing a graded core/shellsemiconductor nanorod of claim 1, wherein: the mixture is kept at atemperature of approximately 160° C. for between 5 minutes and 24 hoursafter combining the solution.
 9. The method of growing a gradedcore/shell semiconductor nanorod of claim 1, wherein: the mixture iskept at a temperature of 160° C. for 10 minutes after combining thesolution.
 10. The method of growing a graded core/shell semiconductornanorod 10, wherein: the core is a shaped nanorod.
 11. The method ofgrowing a graded core/shell semiconductor nanorod of claim 10, wherein:the core has a tetrapod shape.
 12. The method of growing a gradedcore/shell semiconductor nanorod of claim 1, wherein: the gradedcore/shell nanorod is photochemically annealed.
 13. The method ofgrowing a graded core/shell semiconductor nanorod of claim 12, wherein:the annealing is done using an Ar+ laser.
 14. The method of growing agraded core/shell semiconductor nanorod of claim 1, wherein: the corecomprises CdSe and the graded shell comprises CdS/ZnS.